Fluid ejection device with mixing beads

ABSTRACT

In an embodiment, a fluid ejection device includes a die substrate with a chiclet adhered by its front side to the die substrate. The fluid ejection device also includes an ink delivery slot formed through the chiclet from its back side to its front side. The fluid ejection device further includes a mixing bead at the back side of the chiclet, adjacent the ink delivery slot.

BACKGROUND

Inkjet printheads are non-contact fluid ejection devices that eject inkfrom printhead nozzles onto a media substrate (e.g. paper) to form animage. Thermal inkjet printheads eject drops from a nozzle by passingelectrical current through a heating element to generate heat andvaporize a small portion of the fluid ink within a firing chamber.Piezoelectric inkjet printheads use a piezoelectric material actuator togenerate pressure pulses that force ink drops out of a nozzle. Whileboth dye-based and pigment-based inks are used in inkjet printheads,properties such as color, jettability, drying time, long term storagestability, and decap time (the amount of time a printhead can be leftuncapped and idle and can still fire ink droplets properly), influencewhich type of ink is used in a particular printhead.

Pigment-based inks are increasingly used over dye-based inks because ofthe various advantages they provide, such as color strength and waterfastness. Pigment particles are larger and remain in suspension ratherthan dissolving in liquid. This provides greater color intensity as thepigment inks remain more on the surface of the paper instead of soakinginto the paper. Pigment inks also tend to be more durable and permanentthan dye inks. For example, pigment inks smear less than dye inks whenthey encounter water.

Unfortunately, pigments (colorant particles) suspended in the inkvehicle/carrier tend to settle when a printhead is not used for anextended period of time. Pigment settling can cause printhead nozzles toclog, which reduces the overall print quality.

BRIEF DESCRIPTION OF THE DRAWINGS

The present embodiments will now be described, by way of example, withreference to the accompanying drawings, in which:

FIG. 1 a shows a fluid ejection system implemented as an inkjet printingsystem, according to an embodiment;

FIG. 1 b shows a perspective view of an example inkjet cartridge thatincludes an inkjet printhead assembly and ink supply assembly, accordingto an embodiment;

FIG. 2 shows a cross-sectional side view of an example inkjet cartridgethat includes a printhead with mixing beads, according to an embodiment;

FIG. 3 shows a cross-sectional view of the printhead cutout from FIG. 2,according to an embodiment;

FIGS. 4 and 5 show cross-sectional side views of example inkjetcartridges where mixing beads are experiencing different bead rasteringmodes, according to embodiments;

FIGS. 6 and 7 show cross-sectional side views of example inkjetcartridges where magnetic mixing beads are experiencing different beadrastering modes using a single electromagnet, according to embodiments;

FIGS. 8 and 9, show flowcharts of example methods related to a fluidejection device with mixing beads and electromagnets that function todisrupt pigment settling within the printhead fluid ejection device,according to embodiments.

DETAILED DESCRIPTION Overview

As noted above, while the use of pigment-based inks in inkjet printheadsprovides certain advantages, there are also challenges with their use.When there are extended periods of time when a printhead is inactive,high pigment load and/or settling-prone inks demonstrate a settlingdynamic referred to as PIVS (Pigment Ink Vehicle Separation) that canalter the local composition of ink volumes within the printhead nozzles,firing chambers, and in some cases, beyond an inlet pinch toward theshelf/trench (ink slot) interface. In addition to PIVS, anevaporation-driven “thickening” or “hardening” of ink can occur withinthe bore/nozzle (and in some cases within the chamber as well) due tothe depletion of in-ink water molecules and the subsequent elevation inthe local ink viscosity. Following periods of nozzle inactivity, thevariation in properties of these localized volumes can modify dropejection dynamics (e.g., drop trajectories, velocities, shapes andcolors). When printing resumes after an inactive, non-jetting period,there is an inherent delay before the local ink volumes within thenozzle bores are refreshed. This delay, and the associated effects ondrop ejection dynamics following a non-jetting period, can becollectively referred to as decap response.

Prior methods of mitigating decap response have focused mostly on inkformulation chemistries, minor architecture adjustments, tuning nozzlefiring parameters, and/or servicing algorithms. These approaches haveoften been directed toward specific printer/platform implementations,however, and have therefore not provided a universally suitablesolution.

Efforts to mitigate the decap response through adjustments in inkformulation, for example, often rely on the inclusion of key additivesthat offer benefits only when paired with specific dispersionchemistries. Architecture focused strategies have typically leveragedshortened shelves (i.e., the length from the center of the firingresistor to the edge of the incoming ink-feed slot), the inclusion orexclusion of counter bores, and modifications to resistor sizes. Thesetechniques, however, usually provide only minimal performance gains.Fire pulse routines have shown some improvements in targetedarchitectures when exercised as sub-TOE (turn on energy) mixingprotocols for stirring ink within the nozzle to combat PIVS forms of thedecap dynamic, or by delivering more energetic stimulation of in-chamberink volumes (delivered at higher voltages or through modified precursorpulse configurations) to compete against viscous plugging forms of thedecap response. Again, however, this strategy provides only marginalgains in specific non-universal contexts. Servicing algorithms havefunctioned as the main systems-based fix. However, servicing algorithmstypically generate waste ink and associated waste ink storage issues,in-printer aerosol, and print/wipe protocols that are only feasible forimplementation as pre- or post-job exercises.

Another technique for mitigating decap response issues involves“outrunning” the settling and thickening of ink through continuedprinting. This technique is often a viable choice in high-throughputapplications where a printer (e.g., a large format, fixed printbarprinting system) is heavily utilized in a consistent and regular way.Unfortunately, it is not always the case that such use modes can beexpected, and the penalties associated with settling-prone inks increasesignificantly as other use modes are employed.

More recent solutions include nozzle-level micro-recirculationstrategies, as well as macro-recirculation strategies that focus onstimulating fluid flow behind the back-side of the printhead die.Challenges with micro-recirculation designs include difficulties inhomogenizing ink volumes that are upstream of the printhead die, whichunfortunately can permit pigment settling in other regions of theprinthead that are important for delivering fresh ink. Conversely,challenges with macro-recirculation designs often include pigmentsettling in smaller regions and regions where the flow follows sharpturns within the printhead. Once settling begins in such areas, it cancascade into other parts of the ink delivery system.

Embodiments of the present disclosure provide significant improvementover prior efforts to mitigate decap response issues, especially withregard to the complex issue of PIVS (Pigment Ink Vehicle Separation)associated with high pigment load and/or settling-prone inks. Aprinthead fluid ejection device includes bead-like structures such asball bearings in the ink delivery system (IDS) immediately upstream ofthe chiclet die carrier. Periodically rastering these mixing beads backand forth along the elongated axis of the chiclet ink delivery slots(one bead per slot) disrupts the settling dynamic and subsequent nozzlefouling complications typically observed with such inks. Entrainmenteffects of the rastering beads create a mixing dynamic that canre-suspend settled pigments. The beads operate to mix fluid down toregions of the die close to the jetting nozzles, and can also introducemixing flows that propagate effectively into the larger upstream IDSgeometry. The rastering response can be implemented, for example,through the use of small electromagnets positioned within the printheadat opposing ends of the chiclet ink delivery slots. Metal (e.g.,ferrous-core) beads can be rastered by actuating the electromagnets atopposing ends of the chiclet, 180 degrees out of phase. The couplingbetween the beads and the magnetic field can be amplified (madestronger) by using a magnet as the bead. In this case, theelectromagnets at each end of the chiclet slot can work in combination,and simultaneously, with an electromagnet at one end of the slot pushingthe bead magnet away while the electromagnet at the other end of theslot draws the bead magnet near. In a further implementation, a singleelectromagnet on one end of the chiclet can perform the rastering of abead magnet by shifting its polarity through current reversal throughthe coil. Such a configuration enables this technology to more easilyfit into varying printhead form factors.

In an example embodiment, a fluid ejection device includes a diesubstrate. A chiclet is adhered to the die substrate at its front side.An ink delivery slot is formed through the chiclet from its back side toits front side. A mixing bead is installed at the back side of thechiclet, adjacent the ink delivery slot. In other embodiments, the fluidejection device includes an electromagnet to raster the bead back andforth across the ink delivery slot.

In another example embodiment, a processor-readable medium stores coderepresenting instructions that when executed by a processor cause theprocessor to turn on first and second electromagnets in a fluid ejectiondevice to raster a mixing bead back and forth across an ink deliveryslot, wherein the first electromagnet is located at a first side of theink delivery slot and the second electromagnet is located at a secondside of the ink delivery slot.

In another example embodiment, a processor-readable medium stores coderepresenting instructions that when executed by a processor cause theprocessor to turn on a single electromagnet located at a first side ofan ink delivery slot in a fluid ejection device, such that the singleelectromagnet has a first polarity, and turn on the single electromagnetsuch that the single electromagnet has a reverse polarity.

Illustrative Embodiments

FIG. 1 a illustrates a fluid ejection system implemented as an inkjetprinting system 100, according to an embodiment of the disclosure.Inkjet printing system 100 generally includes an inkjet printheadassembly 102, an ink supply assembly 104, a mounting assembly 106, amedia transport assembly 108, an electronic controller 110, and at leastone power supply 112 that provides power to the various electricalcomponents of inkjet printing system 100. In this embodiment, fluidejection devices 114 are implemented as fluid drop jetting printheads114. Inkjet printhead assembly 102 includes at least one fluid dropjetting printhead 114 that ejects drops of ink through a plurality oforifices or nozzles 116 toward print media 118 so as to print onto theprint media 118. Nozzles 116 are typically arranged in one or morecolumns or arrays such that properly sequenced ejection of ink fromnozzles 116 causes characters, symbols, and/or other graphics or imagesto be printed on print media 118 as inkjet printhead assembly 102 andprint media 118 are moved relative to each other. Print media 118 can beany type of suitable sheet or roll material, such as paper, card stock,transparencies, Mylar, and the like. As further discussed below, eachprinthead 114 comprises one or more mixing beads 117 and electromagnets119 that function in varying implementations to effect a disruption of aPIVS settling dynamic that maintains and/or restores local ink volumeswithin the printhead fluid ejection device according to their naturalsuspended compositions.

Ink supply assembly 104 supplies fluid ink to printhead assembly 102 andincludes a reservoir 120 for storing ink. Ink flows from reservoir 120to inkjet printhead assembly 102. Ink supply assembly 104 and inkjetprinthead assembly 102 can form either a one-way ink delivery system ora macro-recirculating ink delivery system. In a one-way ink deliverysystem, substantially all of the ink supplied to inkjet printheadassembly 102 is consumed during printing. In a macro-recirculating inkdelivery system, however, only a portion of the ink supplied toprinthead assembly 102 is consumed during printing. Ink not consumedduring printing is returned to ink supply assembly 104.

In some implementations, as shown in FIG. 1 b, inkjet printhead assembly102 and ink supply assembly 104 (including reservoir 120) are housedtogether in a replaceable device such as an integrated inkjet printheadcartridge or pen 103. FIG. 1 b shows a perspective view of an exampleinkjet cartridge 103 that includes inkjet printhead assembly 102 and inksupply assembly 104, according to an embodiment of the disclosure. Inaddition to one or more printhead dies 114, inkjet cartridge 103includes electrical contacts 105 and an ink (or other fluid) supplychamber 107. Electrical contacts 105 carry electrical signals to andfrom controller 110, for example, to cause the ejection of ink dropsthrough nozzles 116. Cartridge 103 can have a single supply chamber 107that stores one color of ink, or a number of chambers 107 that eachstore a different color of ink. In some implementations, a largerreservoir may also be located separately from the cartridge 103 torefill the local chamber 107 through an interface connection, such as asupply tube. In various implementations, cartridge 103 and/or reservoir120 of ink supply assembly 104 may be removed, replaced, and/orrefilled.

Mounting assembly 106 positions inkjet printhead assembly 102 relativeto media transport assembly 108, and media transport assembly 108positions print media 118 relative to inkjet printhead assembly 102.Thus, a print zone 122 is defined adjacent to nozzles 116 in an areabetween inkjet printhead assembly 102 and print media 118. In oneimplementation, inkjet printhead assembly 102 is a scanning typeprinthead assembly. As such, mounting assembly 106 includes a carriagefor moving inkjet printhead assembly 102 relative to media transportassembly 108 to scan print media 118. In another implementation, inkjetprinthead assembly 102 is a non-scanning type printhead assembly. Assuch, mounting assembly 106 fixes inkjet printhead assembly 102 at aprescribed position relative to media transport assembly 108. Thus,media transport assembly 108 positions print media 118 relative toinkjet printhead assembly 102.

In one implementation, inkjet printhead assembly 102 includes oneprinthead 114. In another implementation, inkjet printhead assembly 102is a wide-array assembly with multiple printheads 114. In wide-arrayassemblies, an inkjet printhead assembly 102 typically includes acarrier that carries printheads 114, provides electrical communicationbetween the printheads 114 and electronic controller 110, and providesfluidic communication between the printheads 114 and ink supply assembly104.

In one implementation, inkjet printing system 100 is a drop-on-demandthermal bubble inkjet printing system where the printhead(s) 114 is athermal inkjet (TIJ) printhead. The TIJ printhead employs a thermalresistor ejection element in an ink chamber to vaporize ink and createbubbles that force ink or other fluid drops out of a nozzle 116. Inanother implementation, inkjet printing system 100 is a drop-on-demandpiezoelectric inkjet printing system where the printhead(s) 114 is apiezoelectric inkjet (PIJ) printhead that implements a piezoelectricmaterial actuator as an ejection element to generate pressure pulsesthat force ink drops out of a nozzle.

Electronic controller 110 typically includes one or more processors 111,firmware, software, one or more computer/processor-readable memorycomponents 113 including volatile and non-volatile memory components(i.e., non-transitory tangible media), and other printer electronics forcommunicating with and controlling inkjet printhead assembly 102,mounting assembly 106, and media transport assembly 108. Electroniccontroller 110 receives data 124 from a host system, such as a computer,and temporarily stores data 124 in a memory 113. Typically, data 124 issent to inkjet printing system 100 along an electronic, infrared,optical, or other information transfer path. Data 124 represents, forexample, a document and/or file to be printed. As such, data 124 forms aprint job for inkjet printing system 100 and includes one or more printjob commands and/or command parameters.

In one implementation, electronic printer controller 110 controls inkjetprinthead assembly 102 to eject ink drops from nozzles 116. Thus,electronic controller 110 defines a pattern of ejected ink drops thatform characters, symbols, and/or other graphics or images on print media118. The pattern of ejected ink drops is determined, for example, by theprint job commands and/or command parameters from data 124.

In one implementation, electronic controller 110 includes a beadrastering module 128 stored in a memory 113 of controller 110. Beadrastering module 128 includes coded instructions executable by one ormore processors 111 of controller 110 to cause the processor(s) 111 toimplement various rastering routines to control electromagnets within aprinthead 114 to effect the rastering back and forth of mixing beads 117along the elongated axis of chiclet ink delivery slots within theprinthead 114, as discussed more fully below.

FIG. 2 shows a cross-sectional side view of an example inkjet cartridge103 that includes a printhead 114 with mixing beads 117, according to anembodiment of the disclosure. FIG. 3 shows a cross-sectional view of theprinthead 114 cutout 200 from FIG. 2. Referring to FIGS. 2 and 3, themixing beads 117 are located in printhead 114 adjacent to ink deliveryslots 202 (one bead per slot) on the back side of chiclet 204. Ingeneral, the beads are sized large enough that they cannot slip downinto ink delivery slots 202 of the chiclet 204. As can be seen moreclearly in FIG. 3, chiclet 204 is the printhead die substrate 206carrier, and it includes carrier ribs 208 which define the chiclet inkdelivery slots 202 (i.e., the fluid passageways within the chiclet). Thechiclet 204 is a fluid distribution manifold such as a plastic fluidicinterposer whose ink delivery slots 202 provide fluid passagewaysbetween the plastic housing 300 of cartridge 103 and the printhead diesubstrate 206. While only two slots 202 are illustrated and discussed,it should be apparent that the concepts disclosed herein apply equallyto printhead configurations in which a chiclet has varying numbers ofslots 202. The printhead substrate 206 is typically fabricated from asilicon or glass wafer through standard micro-fabrication processes suchas electroforming, laser ablation, etching, sputtering, dry etching,photolithography, casting, molding, stamping, machining, and so on. Theprinthead substrate 206 is also further developed to include a fluidicsand nozzle layer 302 on a top side of the substrate 206. Adhesive bonds304 generally adhere substrate 206 to the carrier ribs 208 at the frontside of chiclet 204, and adhere the back side of chiclet 204 to theplastic housing 300 of cartridge 103.

As beads 117 raster back and forth along the elongated axis of chiclet204 ink delivery slots 202 within the printhead 114, they create a fluidmixing dynamic 210 that re-suspends pigments that have settled out ofthe fluid ink vehicle. The beads 117 operate to mix fluid down toregions of the substrate 206 close to the jetting nozzles 116 of nozzlelayer 302, and can also introduce mixing flows that propagateeffectively into the larger upstream IDS geometry within the plastichousing 300 of cartridge 103.

While moving the cartridge 103 back and forth (e.g., by shaking itmanually) can effectively raster the beads 117 back and forth within theprinthead 114 to achieve fluidic mixing, automated processes ofrastering of the beads 117 are also possible. FIGS. 4 and 5 show across-sectional side view of an example inkjet cartridge 103 where themixing beads 117 are experiencing different bead rastering modes,according to embodiments of the disclosure. In the implementations ofFIGS. 4 and 5, the mixing beads 117 are metal beads, formed of aferromagnetic material, such as ferrous-core beads. The beads 117 inFIGS. 4 and 5 can also be formed of other ferromagnetic materials suchas nickel and cobalt. In addition, beads 117 may be coated with aprotective layer that protects them from the corrosive effects of ink,such as a polymer layer.

Because beads 117 are formed of a ferromagnetic material, they areresponsive to the forces of magnetic fields, which can attract and repelsuch materials. Accordingly, printhead 114 can be equipped with one ormore electromagnets 400 positioned within the printhead 114 at opposingends of the chiclet ink delivery slots 202. Electromagnets 400 generallycomprise a coil of wire wrapped around a core of ferromagnetic materialsuch as steel. An electromagnet 400 acts as a magnet when an electriccurrent passes through the coil, and ceases acting as a magnet when thecurrent stops. The ferromagnetic core around which the coil is wrappedenhances the magnetic field produced by the coil.

Electric current (e.g., from a power supply 112) passing through thecoils of electromagnets 400 is controllable by a processor 111 executinginstructions from a bead rastering module 128 stored in a memory 113.Thus, the processor 111 controls when the electromagnets 400 turn ON,and when they turn OFF, to control when and how the beads 117 arerastered back and forth across the ink delivery slots 202 of chiclet 204within the printhead 114. For example, as shown in FIGS. 4 and 5, theprocessor 111 can raster the beads 117 back and forth by actuating theelectromagnets 400 (400 a and 400 b) at opposing ends of the chiclet204, 180 degrees out of phase with one another. In FIG. 4, anelectromagnet 400 a at one end of the chiclet 204 (i.e., on the rightside) is turned ON by processor 111, which pulls the bead to the right,toward the electromagnet 400 a. At this time, the electromagnet 400 b(i.e., on the left side) is OFF. This raster mode allows the bead(s) 117to move to the right and traverse the length of the slot 202.Thereafter, as shown in FIG. 5, the electromagnet 400 b at the other endof the chiclet 204 (i.e., on the left side) is turned ON by processor111, while the electromagnet 400 a is turned OFF. This raster mode pullsthe bead(s) 117 back across the slot 202 to the left, toward theelectromagnet 400 b.

In another implementation of the printhead 114 configuration shown inFIGS. 4 and 5, the beads 117 can be magnets. That is, the beads 117 areformed of material that is magnetized and creates its own persistentmagnetic field. When beads 117 are magnets, the magnetic couplingbetween the beads 117 and electromagnets 400 is amplified. By theprocessor 111 alternately shifting the polarity of the electromagnets400 through reversing the direction of current through the coils, theelectromagnets 400 at each end of the slot 202 can work simultaneouslyand in combination to move the beads 117 back and forth across the slots202. That is, for example, while electromagnet 400 a is ON in onepolarity (e.g., a positive polarity), electromagnet 400 b is ON in thereverse polarity (e.g., a negative polarity). In this mode,electromagnet 400 a will pull magnetic bead 117 to the right, whileelectromagnet 400 b pushes magnetic bead 117 to the right. After themagnetic bead 117 reaches the right side of the slot 202, processor 111can control a reversal of the direction the current flows through thecoils of electromagnets 400 a and 400 b, thereby reversing theirpolarities. In this mode, electromagnet 400 a will push magnetic bead117 to the left, while electromagnet 400 b pulls magnetic bead 117 tothe left.

FIGS. 6 and 7 show a cross-sectional side view of an example inkjetcartridge 103 where magnetic mixing beads 117 are experiencing differentbead rastering modes using a single electromagnet, according toembodiments of the disclosure. In the implementations of FIGS. 6 and 7,the mixing beads 117 are formed of magnetized material, such that theycreate their own magnetic fields. Materials that can be magnetizedinclude, for example, various ferromagnetic materials such as iron,nickel, cobalt, some metal alloys, and some naturally occurring mineralssuch as lodestone.

The bead rastering modes illustrated in FIGS. 6 and 7 are achieved withthe use of a single electromagnet 400 on one end of the chiclet 204 inkdelivery slots 202. The polarity of the single electromagnet 400 isalternately shifted through current reversal through the coil. As shownin FIG. 6, a barrier 600 in the printhead 114 maintains the orientationof the polarized magnetic bead 117. In the raster mode show in FIG. 6,the processor 111 controls current flow through the coil ofelectromagnet 400 so that it generates a south (S) polarized magneticfield. The magnetic bead 117 is oriented such that its south (S) pole istoward the electromagnet 400, which causes the electromagnet 400 torepel the magnetic bead 117, moving it toward the right side of the slot202. In the raster mode show in FIG. 7, the processor 111 reverses thedirection of current flow through the coil of electromagnet 400 so thatit generates a north (N) polarized magnetic field. Because the magneticbead 117 is oriented such that its south (S) pole is toward theelectromagnet 400, the electromagnet 400 pulls on the magnetic bead 117,moving it toward the left side of the slot 202. The use of a singleelectromagnet 400 to raster the magnetic beads 117 back and forth acrossthe chiclet slots 202 improves the likelihood that such technology canbe fit into additional printhead form factors that have tighter spacerestrictions.

FIGS. 8 and 9, show flowcharts of example methods 800 and 900, relatedto a fluid ejection device (e.g., a printhead) with mixing beads andelectromagnets that function to disrupt pigment settling within theprinthead fluid ejection device, according to embodiments of thedisclosure. Methods 800 and 900 are associated with the embodimentsdiscussed above with regard to FIGS. 1-7, and details of the steps shownin methods 800 and 900 can be found in the related discussion of suchembodiments. The steps of methods 800 and 900 may be embodied asprogramming instructions stored on a computer/processor-readable medium,such as memory 113 of FIG. 1. In an embodiment, the implementation ofthe steps of methods 800 and 900 are achieved by the reading andexecution of such programming instructions by a processor, such asprocessor 111 of FIG. 1. Methods 800 and 900 may include more than oneimplementation, and different implementations of the methods 800 and 900may not employ every step presented in their respective flowcharts.Therefore, while steps of methods 800 and 900 are presented in aparticular order within the flowcharts, the order of their presentationis not intended to be a limitation as to the order in which the stepsmay actually be implemented, or as to whether all of the steps may beimplemented. For example, one implementation of method 800 might beachieved through the performance of a number of initial steps, withoutperforming one or more subsequent steps, while another implementation ofmethod 800 might be achieved through the performance of all of thesteps.

Method 800 of FIG. 8, begins at block 802, where the first step shown isto turn on first and second electromagnets in a fluid ejection device toraster a mixing bead back and forth across an ink delivery slot. In thisstep, the first electromagnet is located at a first side of the inkdelivery slot and the second electromagnet is located at a second sideof the ink delivery slot. As shown at blocks 804, 806, and 808,respectively, turning on the first and second electromagnets can includeturning on the first electromagnet, turning off the first electromagnet,and, upon turning off the first electromagnet, turning on a secondelectromagnet. As shown at block 810, where the mixing bead is a magnet,turning on the first and second electromagnets can include turning onthe first and second electromagnets simultaneously such that the firstelectromagnet pulls the mixing bead in a first direction while thesecond electromagnet pushes the mixing bead in the first direction.

Method 900 of FIG. 9, begins at block 902 where the first step shown isto turn on a single electromagnet such that the single electromagnet hasa first polarity. The single electromagnet is located at a first side ofan ink delivery slot in a fluid ejection device. Turning on the singleelectromagnet includes applying electric current to a coil of theelectromagnet in a first direction. The next step in method 900, asshown at block 904, is to turn on the single electromagnet such that thesingle electromagnet has a reverse polarity (i.e., an opposite polarityfrom the first polarity). Turning on the single electromagnet such thatthe single electromagnet has a reverse polarity includes applying theelectric current to the coil in a reverse direction.

What is claimed is:
 1. A fluid ejection device comprising: a diesubstrate; a chiclet adhered by its front side to the die substrate; anink delivery slot formed through the chiclet from its back side to itsfront side; and a mixing bead at the back side of the chiclet, adjacentthe ink delivery slot.
 2. A fluid ejection device as in claim 1, furthercomprising two electromagnets, one on either side of the ink deliveryslot to raster the mixing bead back and forth across the ink deliveryslot through alternating activation of the two electromagnets.
 3. Afluid ejection device as in claim 1, wherein the mixing bead comprises amagnet, the fluid ejection device further comprising two electromagnets,one on either side of the ink delivery slot to raster the mixing beadback and forth across the ink delivery slot through simultaneousactivation of the two electromagnets.
 4. A fluid ejection device as inclaim 1, further comprising a single electromagnet on one side of theink delivery slot to raster the mixing bead back and forth across theink delivery slot through reversing a direction of current flow througha coil of the electromagnet.
 5. A fluid ejection device as in claim 3,wherein simultaneous activation of the two electromagnets comprisesalternating the polarities of the two electromagnets with eachactivation.
 6. A fluid ejection device as in claim 1, wherein the mixingbead comprises a metal bead.
 7. A fluid ejection device as in claim 6,wherein the metal bead is formed of a ferromagnetic material selectedfrom the group consisting of iron, nickel, cobalt, and metal alloy.
 8. Afluid ejection device as in claim 1, wherein the mixing bead comprises amagnet.
 9. A fluid ejection device as in claim 1, wherein the mixingbead is sized such that it cannot enter the ink delivery slot.
 10. Afluid ejection device as in claim 1, further comprising a polymer layercoating the mixing bead.
 11. A processor-readable medium storing coderepresenting instructions that when executed by a processor cause theprocessor to: turn on first and second electromagnets in a fluidejection device to raster a mixing bead back and forth across an inkdelivery slot; wherein the first electromagnet is located at a firstside of the ink delivery slot and the second electromagnet is located ata second side of the ink delivery slot.
 12. A processor-readable mediumas in claim 11, wherein turning on the electromagnets comprises: turningon the first electromagnet; turning off the first electromagnet; andupon turning off the first electromagnet, turning on a secondelectromagnet.
 13. A processor-readable medium as in claim 11, whereinthe mixing bead is a magnet, and turning on the electromagnets comprisesturning on the first and second electromagnets simultaneously such thatthe first electromagnet pulls the mixing bead in a first direction whilethe second electromagnet pushes the mixing bead in the first direction.14. A processor-readable medium storing code representing instructionsthat when executed by a processor cause the processor to: turn on asingle electromagnet located at a first side of an ink delivery slot ina fluid ejection device, such that the single electromagnet has a firstpolarity; and turn on the single electromagnet such that the singleelectromagnet has a reverse polarity.
 15. A processor-readable medium asin claim 14, wherein: turning on the single electromagnet to have thefirst polarity comprises applying electric current to a coil of theelectromagnet in a first direction; and turning on the singleelectromagnet to have the reverse polarity comprises applying theelectric current to the coil in a reverse direction.